The presence of dominant transforming genes in cell lines derived from human tumors and in human cell lines that have been transformed by chemical carcinogens in vitro has been investigated. High molecular weight DNA samples prepared from a teratocarcinoma cell line, a fibrosarcoma cell line and from human cells that were transformed by MNNG were applied to mouse NIH3T3 cells using the calcium phosphate precipitation technique. After transfection the cells were either reseeded and scored for foci of transformed cells after 10-14 days or injected into athymic nude mice, which were examined periodically for the presence of tumors. The results obtained show that the DNA prepared from the MNNG transformed cells and the teratocarcinoma cells were able to morphologically transform NIH3T3 cells and that cells transfected with DNA from the MNNG transformed cells and the fibrosarcoma give rise to tumors in nude mice. Both the morphologically transformed NIH3T3 cells and the tumors possessed DNA highly repeated human Alu sequences.